1. Field of the Invention
The present invention relates to methods of reception of a coded data signal by a receiver and, more particularly, to methods for producing and providing assisting signals to a demodulator in the receiver to improve decoding of the received coded data signal.
2. Description of Related Art
In data signal transmission, coding is often employed, for instance to enable reducing the required transmission energy per data bit for the same error rate. The main task of a data decoder downstream of a demodulator in the receiver is to determine a train of data bits d'.sub.k from the train of estimated values c'.sub.i, possibly provided with quality standards, for the channel symbols c.sub.i actually sent, so that the error probabilities of these data bits d'.sub.k, referred to the actual data bits d.sub.k of the source, are as low as possible.
One example of such a decoder is the one described in reference 1 (paper by Forney, "The Viterbi Algorithm", Proceedings of the IEEE, Volume 61, No. 3, March 1973, pages 268-278), which is also known as a Viterbi decoder.
A second task of the data decoder can be to support the demodulator in determining the estimated values c'.sub.i from the received signal r(t). One example of such support is described in reference 2 (paper by Ungerboeck, "Channel Coding with Multilevel/Phase Signals"), IEEE IT-28, No. 1, January 1982, pages 55-67, especially FIG. 17). In addition to the train of data bits d'.sub.k, the data decoder determines a train of improved estimated values c".sub.i for the channel symbols, but this train is delayed compared to the train of estimated values c'.sub.i furnished by the demodulator. The estimated values for the channel symbols furnished by the channel symbols are also known as tentative decisions. The feedback of the estimated values c".sub.i to the demodulator, however, creates a closed-loop control circuit, or control loop, in which the aforementioned delay is problematic. The feedback also creates problems in the initial synchronization, because the closed-loop control circuit under some circumstances can remain for relatively long periods in unstable states of equilibrium, known as "false locks".